Conductive fine particles are widely used as anisotropic conductive materials such as anisotropic conductive paste, anisotropic conductive ink, anisotropic conductive adhesive, anisotropic conductive film, and anisotropic conductive sheet by being mixed or kneaded with binder resin, adhesives and the like.
These anisotropic conductive materials are used while being sandwiched between opposing substrates or electrode terminals, for example, for electrically connecting the substrates, or for electrically connecting a small part such as a semiconductor device to a substrate, in electronic devices such as a liquid crystal display, a personal computer, and a cellular phone.
As the conductive fine particles used in such anisotropic conductive materials, conductive fine particles in which metal plating layers are formed as conductive films on the surfaces of non-conductive fine particles such as resin fine particles having uniform particle size and appropriate strength have been conventionally used. However, with the recent rapid advance and development in electronic devices, there arises a need to further reduce connection resistance of conductive fine particles used as an anisotropic conductive material.
In order to reduce the aforementioned connection resistance of conductive fine particles, conductive fine particles having protrusions on the surface are reported (see Patent document 1, for example). Also, conductive fine particles having protrusions on the surface and provided with an insulation layer on the circumference of particle are reported (see Patent document 2, for example).
Patent document 1 discloses the conductive fine particles in which micro protrusions are formed on metal plated surface by use of abnormal deposition phenomenon in plating reaction when electroless metal plating is affected on the surface of resin fine particles. Since the protrusions have almost the same hardness as that of electrodes, there is little possibility that the protrusions break the electrodes. However, with respect to protrusions formed by the abnormal deposition phenomenon method, since protrusions are formed according to plating condition, it is difficult to sufficiently ensure the conductivity because there is limitation in density and size for providing protrusions having enough adhesiveness to crash through the binder resin of an anisotropic conductive film.
Therefore, in order to ensure high connection reliability, it is necessary to increase a blending amount of conductive fine particles in an anisotropic conductive material. However, increase in the blending amount will result in horizontal conduction between adjacent conductive fine particles, for example, in a substrate having fine wiring, which may cause the problem of occurrence of short between adjacent electrodes. In particular, leak current caused by conductive fine particles becomes problematic as pitch of electrode becomes finer in recent years.
Patent document 2 discloses conductive silica-based particles in which a conductive coating layer is formed on silica-based base particles having protrusions in the entire face of base particles, and having different hardness from the protrusions, and conductive fine particles further formed with an insulation layer on outer peripheries of the silica-based particles. However, since silica particles used in the base particles and protrusions are hard, there is a fear that pressure at the time of pressure bonding may break electrodes when such particles are used as an anisotropic conductive material such as anisotropic conductive film.
Patent document 1: Japanese Unexamined Patent Application No. 2000-243132
Patent document 2: Japanese Unexamined Patent Application No. 2004-35293